This invention relates to HID sodium lamps and, more particularly, to a method for operating such lamps in order to minimize lamp voltage variations throughout lamp life.
It is well known to modify the performance of high-intensity-discharge (HID) lamps by sensing a lamp operating parameter and controlling the lamp operation in accordance with this sensed parameter. One such modifying device is disclosed in aforementioned application Ser. No. 282,993, filed Jan. 14, 1981 now abandoned. In its preferred form, a series-connected additional inductor and a gate-controlled AC switch are connected in parallel across the capacitor of the lead-type ballast and a sensing and programming means operates to sense at least one lamp oprating parameter in order to control the proportion of time the AC switch is open and closed, in order to vary the current input to the lamp. The programming and control device of aforementioned copending application Ser. No. 414,115, filed concurrently herewith is particularly adapted to operate with such a modified lead ballast and this improved programming device has specifically been designed to operate an HID sodium lamp in accordance with the present invention.
Another system for controlling lamp wattage is disclosed in U.S. Pat. No. 4,162,429, dated July 24, 1979 to Elms et al. wherein lamp voltage and line voltage are sensed and these parameters are converted into separate signals which are fed into a ramp capacitor to control the charging rate thereof. When the ramp capacitor achieves a predetermined level of charge during each half cycle of AC energizing potential, an AC switch is gated to shift the current level to the operating lamp, in order to control the wattage input thereto. This particular circuit can be modified in order to control the lamp operation so that lamp voltage variations are minimized throughout lamp life.
Some types of HID sodium lamps are designed to operate with an increased loading in order to improve the color rendering of objects illuminated thereby, such as described in U.S. Pat. No. 4,230,964 dated Oct. 28, 1980 to Bhalla. This type of HID sodium lamp tends to display shifts in the operating lamp color temperature with increasing lamp voltage. While the overall performance of the lamp is not affected by such shifts in lamp color temperature, when a series of such lamps are operated in side-by-side fixtures, difference in the color appearance of such proximate lamps can be considered objectionable from an esthetic standpoint.